1. Field of the Invention
The present invention relates to a method and an apparatus for transferring multiple toner images and an image forming apparatus.
2. Description of the Related Art
Image forming apparatus that uses an optical writing, in which a principle of electrophotography is used, has been known so far as a digital copier, an optical printer, an optical plotter, and a facsimile apparatus etc. In these image forming apparatuses, colorization of an image formed has been advancing and there has been a strong demand for speeding up the image formation. Realization of a so called tandem image forming apparatus that enables color image formation at a speed similar to that of monochrome image formation has been intended.
In the tandem image forming apparatus, an electrostatic latent image is formed by optical writing on a plurality of latent image carriers and each electrostatic latent image is developed by a toner of a different color. A toner image of a different color is formed for each latent image carrier. The plurality of toner images is transferred to a common recording medium in the form of a sheet so that they are superimposed mutually, thereby forming a color image or a multicolor image.
Therefore, in the tandem image forming apparatus, a quality of superimposing on one another the plurality of toner images that are transferred to the same recording medium in the form of a sheet ultimately determines a quality of image that is formed. To superimpose accurately the toner images in this manner, as to how to perform with accuracy a so called registering, i.e. matching mutually front tip portions of the toner images becomes an issue of great importance.
In the tandem image forming apparatus, since an optical system that performs optical writing, the latent image carrier, and a transferring section that transfers the toner image are mutually independent, there are many factors such as an amount of distortion of a lens in the optical system and a distance from a writing position to a transfer position that causes a shift of the toner images. Such a shift of the plurality of toner images is called as misalignment.
Moreover, even if it is presumed that the misalignment could be eliminated by adjusting the apparatus perfectly, change in optical characteristics of a resin lens due to a change in temperature, and an expansion and contraction of the latent image carrier etc. is inevitable, and the misalignment of the toner images occurs due to these factors.
As a method to reduce the misalignment, so far, a mark for registration is written as an electrostatic latent image on each latent image carrier and toner images upon visualizing these electrostatic latent images are transferred to a transfer belt. A mutual positional relationship of the mark images is read optically, and based on a result of optical reading, start-timing of the optical writing has been controlled (see, for example, Japanese Patent Application Laid-open Publication No. H8-248721).
In this case, an optical writing unit that performs deflection of a light beam by a rotating polygon mirror is provided for each latent image carrier. A method of performing with high accuracy the registration by controlling with high accuracy the start-timing of the optical writing for each latent image carrier by controlling a phase of rotation of the rotating polygon mirror corresponding to each latent image carrier, has been proposed (see, for example, Japanese Patent Application Laid-open Publication No. H10-138556). However, since the rotation of the rotating polygon mirror being extremely high-speed rotation, it is not easy to control the phase of rotation and necessitates an expensive controlling unit, thereby leading to a rise in cost.
On the other hand, a tandem image forming apparatus in which the rotating polygon mirror is used commonly for the light beams that perform optical scanning of the plurality of latent image carriers has been proposed (Japanese Patent Application Laid-open Publication No. Hei 10-3048. However, in this case, it is not possible to control by corresponding the phase of rotation of the rotating polygon mirror with each latent image carrier and an effect of decreasing the registration has limitations.
Moreover, a method in which a scale is formed on an intermediate transfer belt or a sheet conveying belt to detect a linear velocity, and based on this linear velocity that is detected by detecting this scale by a scale detecting unit, a rotational drive of the intermediate transfer belt or the sheet conveying belt is controlled, has been known (see, for example, Japanese Patent Application Laid-open Publication No. H8-10371).